Much research and development has taken place with solar cells in the last decade. A vast majority of solar cells in the market are silicon-based. In silicon-based solar cells, the silicon materials are in the form of either mono-crystalline or multicrystalline wafers. To get silicon wafers, polysilicon is prepared by chemical vapor deposition (CVD) in reactors from silicon-based chemicals like trichlorosilane (TCS) or saline. Subsequently, the polysilicon is placed in a furnace to achieve crystal growth. After crystal growth, ingots are sliced into wafers, often by using a mechanical slicing technique such as using a wire saw. The wafers are then fabricated into a solar cell. Given that about one-third of the price of wafer-based silicon solar modules is due to the cost of the silicon wafers, using crystalline-silicon material that is thinner than what is currently used can result in notable cost reduction.
The slicing of the wafers to produce thin material layers has its challenges. For example, during the slicing process, as much as 45% of silicon materials may be subject to kerf loss. In addition, producing wafers of desired thickness, such as a wafer that is less than 120 μm, is often difficult with a mechanical slicing technique.
One way to overcome the above disadvantages associated with mechanical slicing is by hydrogen injection into substrates via ion implantation. With this technique, a silicon wafer is subjected to ion implantation and heat treated at around 500° C. The heating causes the hydrogen underneath the surface to form bubbles, which create a weak region in the substrate that eventually facilitates the separating of a thin layer of silicon from the rest of the substrate. This type of “slicing” technique, sometimes referred to as Smart Cut, has been used for semiconductor and Kerfless substrates in solar cell applications.
Another alternative to mechanical slicing is an electrochemical technique. With this technique, the substrate is electrochemically etched to form a porous layer. Typically, two porous layers form on a substrate. The porous layer that is closer to the exposed surface has a lower porosity than the porous layer that lies deeper in the substrate. At high temperature, the silicon atoms in the top layer rearrange themselves and form a monocrystalline layer. Because the lower layer is highly porous, the thin layer of silicon on top can be separated from the substrate. This method may be used in combination with epitaxy. Namely, after porous layers are formed on the substrate, the substrate is subjected to an epitaxial process whereby an epitaxial layer is grown on the top porous layer. The silicon atom rearrangement that took place before the epitaxial process allows the growth of the epitaxial layer on the top porous layer. After the epitaxial process, the epitaxial layer and the top substrate layer may be separated from the substrate.
Yet another alternative to mechanical slicing is stress-spalling, which entails applying a metal paste on top of a silicon substrate and subjecting the substrate to thermal treatment. Due to the difference in thermal expansion coefficients between the metal and the silicon substrate, compressive stress is introduced into the substrate. Facilitated by this stress, a thin layer of silicon can be separated from the substrate by spalling after cool down.
The above-described alternatives to mechanical slicing, as well as some other known methods, have their disadvantages. Some disadvantages may be, for example, use of HF, high cost, and possible metal contamination. Furthermore, unlike spalling off a single layer from an epitaxial layer, the above methods have varying degrees of substrate consumption. Hence, new techniques for separating a thin material layer from a larger wafer or substrate is desired.